Optical coherence analysis relies on the use of the interference phenomena between a reference wave and an experimental wave or between two parts of an experimental wave to measure distances and thicknesses, and calculate indices of refraction of a sample. Optical Coherence Tomography (OCT) is one example technology that is used to perform usually high-resolution cross sectional imaging. It is often applied to imaging biological tissue structures, for example, on microscopic scales in real time. Optical waves are reflected from an object or sample and a computer produces images of cross sections of the object by using information on how the waves are changed upon reflection.
The original OCT imaging technique was time-domain OCT (TD-OCT), which used a movable reference mirror in a Michelson interferometer arrangement. More recently Fourier domain OCT (FD-OCT) has been developed. Two related FD-OCT techniques are time encoded and spectrum encoded OCT. These Fourier domain techniques use either a wavelength swept source and a single detector, sometimes referred to as time-encoded FD-OCT (TEFD-OCT) or swept source OCT, or, alternatively, a broadband source and spectrally resolving detector system, sometimes referred to as spectrum-encoded FD-OCT or SEFD-OCT. These three OCT techniques parallel the three spectroscopy approaches of Fourier Transform spectrometer, a tunable laser spectrometer, and dispersive grating with detector array spectrometer.
These various OCT techniques offer different performance characteristics. FD-OCT has advantages over time domain OCT (TD-OCT) in speed and signal-to-noise ratio (SNR). Of the two Fourier Domain OCT techniques, swept-source OCT or TEFD-OCT has distinct advantages over SEFD-OCT because of its capability of balanced and polarization diversity detection; it has advantages as well for imaging in wavelength regions where inexpensive and fast detector arrays are not available.
TEFD-OCT or swept source OCT has advantages over SEFD-OCT in some additional respects. The spectral components are not encoded by spatial separation, but they are encoded in time. The spectrum is either filtered or generated in successive frequency steps and reconstructed before Fourier-transformation. Using the frequency scanning swept source the optical configuration becomes less complex but the critical performance characteristics now reside in the source and especially its tuning speed and accuracy.
The swept sources for TEFD-OCT systems have been typically tunable lasers. The advantages of tunable lasers include high spectral brightness and relatively simple optical designs. The typical tunable laser is constructed from a gain medium, such as a semiconductor optical amplifier (SOA), and a tunable filter such as a rotating grating, grating with a rotating mirror, or a Fabry-Perot tunable filter. Currently, some of the highest speed TEFD-OCT lasers are based on the laser designs described in U.S. Pat. No. 7,415,049 B1, entitled Laser with Tilted Multi Spatial Mode Resonator Tuning Element, by D. Flanders, M. Kuznetsov and W. Atia. This highly integrated design allows for a short laser cavity that keeps the round-trip optical travel times within the laser short so that the laser is fundamentally capable of high speed tuning Secondly, the use of micro-electro-mechanical system (MEMS) Fabry-Perot tunable filters combines the capability for wide spectral scan bands with the low mass high mechanical resonant frequency deflectable MEMS membranes that can be tuned quickly.
Another swept laser source for OCT is the Frequency Domain Modelocked Laser (FDML) as described in U.S. Pat. No. 7,414,779 B2. FDML lasers use semiconductor optical amplifiers in a very long, kilometer or more, fiber ring cavity that requires polarization control and active length stabilization.
The use of laser-based swept sources, however, does create problems. The instantaneous laser emission is characterized by one or more longitudinal laser cavity modes that simultaneously lase within the passband of the laser's tunable filter. Then as the laser tunes, the power within these modes shifts between the modes and to new cavity modes that see gain as the tunable filter passband shifts. This spectral structure of the laser emission increases relative intensity noise (RIN), which degrades performance of OCT systems. Another problem is that tunable lasers using ubiquitous semiconductor gain media generally only tune well in one direction, i.e., to longer wavelengths. This is due to a nonlinear asymmetric gain effect in semiconductors that is often called the Bogatov effect. With an optical signal in a semiconductor at a given wavelength, optical waves at longer wavelengths will experience slightly higher optical gain, while optical waves at shorter wavelengths will experience slightly lower optical gain. Such asymmetric nonlinear gain distribution creates a preference for dynamic tuning in the longer wavelength direction, or up tuning, where optical gain is slightly higher, while impeding tuning in the shorter wavelength direction.
Another class of swept sources that have the potential to avoid the inherent drawbacks of tunable lasers is filtered amplified spontaneous emission (ASE) sources that combine a broadband light source, typically a source that generates light by ASE, with tunable filters and amplifiers. Some of the highest speed devices based on this configuration are described in U.S. Pat. No. 7,061,618 B2, entitled Integrated Spectroscopy System, by W. Atia, D. Flanders P. Kotidis, and M. Kuznetsov, which describes spectroscopy engines for diffuse reflectance spectroscopy and other spectroscopic applications such as OCT. A number of variants of the filtered ASE swept source are described including amplified versions and versions with tracking filters.
More recently Eigenwillig, et al. have proposed a variant configuration of the filtered ASE source in an article entitled “Wavelength swept ASE source”, Conference Title: Optical Coherence Tomography and Coherence Techniques IV, Munich, Germany, Proc. SPIE 7372, 73720O (Jul. 13, 2009). The article describes an SOA functioning both as an ASE source and first amplification stage. Two Fabry-Perot tunable filters are used in a primary-tracking filter arrangement, which are followed by a second SOA amplification stage.